The present invention is directed to the field of adhesion testers, and is more specifically directed to apparatus for testing the bond strength of materials which is self aligning.
Adhesion testers which test the bond strength of a material have been made in a number of forms which operate in different ways. However, they generally have four steps in common: (1) attaching a fastener to the material to be tested, (2) applying a tensile force to the fasteners along an axis normal to the surface, (3) measuring the maximum tensile force required to cause bond failure, and (4) computing a relative measure of the bond strength by dividing the maximum tensile force by the stressed surface area of the failed bond. Although these steps appear to be simple, they are subject to the flaws and operator biases which can introduce errors into the final measurement.
The major problem encountered with respect to step (1) is the introduction of flaws into the material prior to testing. Surface flaws, particularly sharp scratches, dents, or corners constitute points of high stress concentration that are essentially pre-loaded. Pre-loading can cause premature failure at these points, which can weaken the sample and introduce shock fronts that initiate total failure, in much the same manner as a score cut in a piece of glass. A second problem is that the means for attachment must be adapted to the material and aligned perpendicular with the material surface. The adaptability of the testing apparatus is important in the testing of in-situ materials, particularly material composites having prepared fasteners.
The major problem encountered with respect to step (2) is that of introducing an uneven stress distribution over the bond area being tested. Bond failures will first occur at the areas of highest stress. The neighboring areas thereby become overloaded, resulting in premature failure of the entire sample. The major cause of uneven stress distribution is the application of off-axis loads, particularly at large distances from the surface being tested, and generally is the result of the structure of the testing device itself. The alignment of devices having attached torquing wheels, alignment feet, hand pumps, or unbalanced gimbals depends heavily upon the skill of the operator, and is therefore high susceptible to error.
The major problem encountered with respect to step (3) stems from unknown or uncontrollable variances between the actual force applied and the indicative parameter being measured. The three commonly measured parameters used to determine force are: counter-balanced masses in known gravitational fields, distances (or displacements) in conjunction with known elastic strains, and pressures in conjunction with known areas. Implicit in the measurement of any of these parameters is the assumption of some exact correlation that is both predictable and verifiable. This assumption often is not warranted because the correlation is affected by unrecognized or unmeasurable frictional losses in slidable systems such as weighted pulleys, moving springs, and constrained moving seals, and/or elastic losses and changing areas in deformable pressurized systems such as membranes and diaphragms.
The major problem encountered with step (4) is that the area of the bond failure is unpredictable and will vary from sample to sample. The force required to separate the material therefore is normalized by determining the failed bond area exposed after each test. Different tests can then be compared. This post-test interpretation of the area is normally left to the failure analyst, but some devices have attempted to define the area prior to testing by scoring the perimeter around the sample. This solution is contraindicated because it defeats the in-situ nature of the test, and gives rise to the types of problems and errors associated with step (1).
A common configuration for devices for testing the bond strength of materials is a pneumatic or hydraulic center pull jack having a conventional seal installed in a classical manner which precludes seal extrusion and requires some initial squeeze friction that increases with pressure. Devices using this configuration are shown in U.S. Pat. Nos. 2,113,725 to Goldman and 3,628,378 to Regan, Jr. These devices characterize the prior art for closed pneumatic systems having slidable sealing elements.
Goldman has the primary advantage of being capable of generating a large force along its axis by means of a relatively small pneumatic hand pump. This advantage is offset by several disadvantages. The maximum fluid pressure serves as a measure of the maximum force, but only the pressurized cross-sectional area can be well defined. The sealing elements--a leather cup seal and a common stuffing box--suffer unmitigated losses from initial squeeze friction, pressure dependent compression friction, and high variable slip-stick friction arising from transitions between the appropriate dynamic and static coefficients as the hand pump hesitates between strokes. The resolution of force required by step (3) based on pressure times area is severely compromised by these three frictional losses. Furthermore, the alignment of the axis of the device with the axis normal to the sample depends on the skill of the operator and is therefore subject to all the problems associated with step (2) which lead to premature failures and inconsistencies.
The Regan, Jr. device constitutes an improvement over the Goldman device in that it eliminates one seal (the stuffing box) and reduces the potential slip-stick friction by applying a contiuous flow of pressurized fluid through a flexible conduit. These changes reduce the alignment problems aggravated by the operation of Goldman's hand-pump. However, the Regan, Jr. device has the same initial alignment problem and suffers from the same frictional losses due to initial squeeze and pressure dependent friction of the slidable seal as the Goldman device. The Regan, Jr. patent recognizes the problem, calling the force measurement "nominal" and suggesting that "the device should be calibrated using an accurate standard." (Column 2, lines 50-52.)
A device which eliminates the slidable seals is characterized by U.S. Pat. No. 3,821,892 to Saberg. Saberg replaces the slidable seals with a deformable seal in the form of a diaphragm. The elastic losses, though ever present, are masked by the inclusion of a helical spring. However, this device suffers from a severe balance problem analogous to that of a two pound pot with a ten pound handle; i.e., the device tips over unless loaded. The gimbal then acts as a lever transferring the weight of the handle into a horizontal force on the fastener (called a dolly in the patent). A non-axial force component assures non-axial loads, premature failures, and erroneous results. Also, the diaphragm must be strong enough to prevent its extrusion into the lower chamber. Poor force alignment and elastic losses are inevitable, as are losses due to the helical spring, so that Saberg's device is incapable of accomplishing either step (2) or (3).
A device incorporating both an annular diaphragm and an annular piston is exemplified by Great Britain Pat. No. 1,455,534 to Centrum Techniki Okretowej. The annular diaphragm is secured to a plate by two annular rings forming a channel which accommodates the annular piston. When in operation, fluid pressure is contained between the plate and the diaphragm. The diaphragm is externally constrained by the inner and outer walls of the securing rings and the top surface of the annular piston. The clearance between the piston and the securing rings determines the elastic properties needed for the diaphragm. If the clearance is made large, so as to impart to the device a self-alignment capability, then the diaphragm will have to be strong enough to prevent its extrusion into the clearance, which could cause it to seize, pinch, or rupture. Conversely, if the clearance is small, then the diaphragm may be made of a thinner material that has a smaller elastic loss and a larger piston contact area. However, the device will then lose the self-alignment capability. Thus, there must be a compromise between the good alignment required by step (2) and the minimal elastic losses and area reductions required by the measurement in step (3). A careful reading of the Centrum patent (page 2, lines 95-96) indicates a preference for minimal losses that precludes the hazards of self-alignment in favor of fixed-alignment of the force "in a direction parallel to the axis of the instrument." The Centrum device is not self-aligning, and the piston may seize in the channel if the axis of both the fastener and the device are not aligned normal to the surface prior to testing.
To reduce elastic losses and obtain self-alignment, it has been proposed to apply pneumatic pressure directly to the test surface and the material fastener. Such a device is exemplified by U.S. Pat. No. 4,393,699 to Seiler, Jr. Seiler, Jr. uses an open-hole membrane peripherally secured to a plate. Air pressure initiates an hermetic seal between the membrane and the material to be tested along a line formed by the membrane hole. The gas is thus contained by four distinct surfaces: the plate, the membrane, the surface of the material being tested, and the material fastener. This device suffers from several problems. First, unmitigated losses still exist along one seal perimeter. Second, the assembly can only test materials that are sufficiently smooth, regular, and non-porous enough to permit an hermetic seal to be formed by the interface between the membrane and the material surface. Third, although this device tends to apply a tensile force in accordance with step (2), the magnitude of that force depends upon the size of the material fastener, or more precisely, upon the pressurized area within the membrane minus the actual area stressed to failure, which can only be determined after each test. Attempts to define the area by scoring the perimeter only violate step (1). Thus, a direct and verifiable force calibration via pressure measurement cannot be assigned to the device without also considering the area to be stressed. Moreover, because the membrane is open at the center, the device does not lend itself easily to direct force calibration techniques using conventional methods. The dependence of the applied tensile force upon the nebulous area stressed by the fastener, and other factors, clearly compromises the force measurement required by step (3). That error in force measurement is compounded by the computation required by step (4).
Another device exemplified by U.S. Pat. No. 4,491,014 to Seiler, Jr., employs a piston having a peripheral groove fitted with a gasket and a central open chamber for the loading fixture, which is subjected to pressure together with the gasket. This device operates according to the same principles as Seiler, Jr. U.S. Pat. No. 4,393,699, and also suffers from the same problems.
In summary, no simple "closed" pneumatic or other pressure-activated system exists for the purpose of testing the bond strength of materials, which is inherently self-aligning as well as being small, inexpensive, and suited to field service work. The "open" pneumatic systems exemplified by the above-described patents are further limited in their application to smooth, regular, non-porous materials, cannot be directly calibrated or verified and produce unpredictable forces depending on the area stressed. It is the solution of these problems to which the present invention is directed.